Electrical circuit



Oct. 25, 1949. w. R. RAMBO 2,485,919: I

ELECTRICAL CIRCUIT Filed Nov. 6, 1945 INVENTOR.

WiLLIAM R. RAMBO -MQAAM ATTORNEY Patented Oct. 25, 1949 ELECTRICALCIRCUIT William R. Rambo, Cambridge, Mass., assignor to the UnitedStates of America as represented by the Secretary of War ApplicationNovember 6, 1945, Serial No. 627,046

12 Claims. (01. 332-28) This application relates to electrical circuitsand particularly to reactance tube circuits.

As known in the art a reactance tube is an electronic device such as avacuum tube in which current flows through the tube out of phase with anapplied radio-frequency voltage. Thus, in a tube of this character, anapplied plate R.-F. voltage may produce a plate current which isapproximately 90 out of phase with the plate voltage, thereby causingthe tube to simulate areactance. Generally this effect is produced byutilizing a phase-splitting network which causes the voltage applied tothe grid of the reactance tube to be in a desired out-of-phaserelationship with the plate voltage. When such an arrangement isassociated with an oscillator tank circuit, it affords a convenientmeans for controlling the oscillator frequency to produce frequencymodulation as a function of a modulating voltage applied to the grid ofthe reactance tube.

In practical applications it is diflicult to maintain the flow ofcurrent through the reactance tube circuit at exactly 90 in phaserelative to the R.-F. voltage throughout the normal range of operatingconditions. This may be attributed to the variation of the tubeplate-cathode impedance with the amplitude of the modulating voltage.Such variation of impedance affects the functioning of thephase-splitting network, and as a result there is apt to be an in-phasecomponent of plate current of variable magnitude which causes a varyingamount of the power of the output of the oscillator to be dissipated inthe reactance tube circuit. The loss of oscillator power as a functionof modulating voltage constitutes a very undesirable form of parasiticamplitude modulation.

Heretofore a number of circuit modifications have been proposed for thepurpose of eliminating the above described parasitic amplitudemodulation. All of these are similar in that an endeavor is made toshift the R.-F. grid voltage more than 90 out of phase with the appliedplate voltage so that, in effect, the reactance tube is capable ofsupplying energy to the system in the proper amount to compensate forits own variable internal losses. Fundamentally, this is a soundsolution to the problem and the present invention utilizes such a methodto accomplish the desired result. However, in prior arrangements, theinternal plate-grid capacitance of the tube has had an undesirableeffect at the higher frequencies. The use of pentode tubes remedies thissituation within the range of frequencies for which pentodes areavailable, but at still higher frequencies, where triodes must beemployed, the plate-grid capacitance is a serious factor which rendersthese circuits unreliable.

A primary object of the present invention is to prevent parasiticamplitude modulation in a reactance tube circuit by the use of acommon-grid reactance tube circuit in which a triode may be utilizedwithout any detrimental effect due to plate-grid capacitance. Acommon-grid circuit, within the meaning of the present specification, isone in which a grid terminal is common to both the input and outputcircuits, in contradistinction to the conventional tube circuit in whichthe cathode terminal is common to both input and output circuits.

A further object is to provide a novel reactance tube circuit of generalapplication at all radio frequencies for achieving the above describeddesired result.

Other objects, features and advantages of the present invention willsuggest themselves to those skilled in the art and will become apparentfrom the following description of the invention taken in connection withthe accompanying drawing which presentsa schematic view of a reactancetube circuit according to the present invention.

Reference is now made to the accompanying drawing in which a basiccommon-grid circuit is illustrated. The reactance tube circuit, in thepresent instance, comprises a triode l, although it should.be understoodthat other tubes such as pentodes may be utilized equally well in thosefrequency ranges where such tube types are available. The plate 2 of thetube l is connected to one of the terminals 3 to which theradio-frequency voltage from the oscillator tank circuit (not shown) isapplied. A phase-splitting network comprising the series combination ofa reactive element 4 and an impedance 5 are connected across the R.-F.terminals 3 and 3'. The cathode 6 of tube I is connected to the junctionof the element 4 and impedance 5. The grid 1 of tube l is connectedthrough a capacitor 8, which presents substantially a short circuit atradio frequencies, to the end of the impedance 5 which is connected tothe R.-F. terminal 3'.

A source 9 of modulating voltage in series with a bias supply I0 isconnected across the capacitor 8, which constitutes substantially anopen circuit at the modulating frequency.

The reactive element 4 may consist of a capacitor II while the impedance5 is a resistor l2. At very high frequencies the capacitor l I may beafforded solely by the plate-cathode capacitance of the tube At thesefrequencies the reactance of the grid-cathode capacitance in tube I maybecome low enough to influence the value of the impedance 5. Preferably,for high-frequency operation, an inductive element such as ahighfrequency line is connected in parallel with the resistor I2 tooffset the capacitive reactance of the grid-cathode capacitance so thatimpedance 5 is substantially a pure resistance. The phasesplittingnetwork tends to displace the radiofrequency voltage applied to the grid1 of the tube substantially 90 electrical degrees with respect to theplate voltage. This phase quadrature relationship is modified by theplate-cathode impedance of tube I which varies with the amplitude of themodulating voltage applied to grid 1.

Because of the interchange of the grid and cathode connections, ascontrasted with onventiona1 circuits, the present arrangement causes theR. F. grid voltage to be displaced more than 90 from the R. F. platevoltage, rather than being otherwise be the case.

The plate current flowing throughout the tube I may be thought of ascomprising two currents,

one produced by application of theR. F, plate voltage irrespective ofthe grid voltage, with variable plate-cathode impedance, and the otherbeing the current produced by the'eifect of the R. F. grid voltage. Thereal components of these two plate currents, that is, those componentswhich are capable of producing power in conjunction with the applied R.F. voltage, are in phase opposition and tend to cancel each other. Inother words, the reactance tube circuit tends to supp energy to theoscillator circuit in an amount which compensates for the dissipation ofoscillator power in the reactance tube circuit.

Actual tests of the above described invention have shown that theloading of the oscillator by the reactance tube is maintainedsubstantially constant over extremely wide modulated bandwidths. Thegeneral simplicity of the circuit and its applicability to triodereactance tubes are es-. pecially advantageous at high frequencies. TheQ, i. e., the ratio of reactance to resistance, of the phase-splittingnetwork can be made very low to minimize any tendency toward parasiticoscillations at possible resonant frequencies that may exist in thereactance tube circuit.

While there has been described what is at present considered to be thepreferred embodiment of the present invention, it will be obvious tothose skilled in the art that various changes and modifications may bemade therein without departing from the scope of the invention.

The invention claimed is:

1. For use in combination with the tank circuit of a radio-frequencyoscillator, a reactance tube circuit comprising a-vacuum tube having aplate, a cathode and a control grid, said plate being directly connectedto one side of said tank circuit, means of negligible impedance at saidfrequency for coupling said grid to the other side of said tank circuit,a reactor connected across said plate and said cathode, an impedanceconnecting said cathode to said other side of said tank circuit, saidreactor and said impedance together comprising a phase-splittingnetwork, and a source of moduating voltage connected across said gridcoupling means.

2. A reactance tube circuit for use in combination with aradio-frequency oscillator, said circuit comprising a two-terminalphase-splitting network, a vacuum tube having a plate, a cathode and acontrol grid, said plate being connected couplings having substantiallylike impedance at.

said frequency, means for applying a potential to the cathode which isphase displaced approximately 90 with respect to the potentials appliedto said grid and anode, and means for varying the transconductance ofsaid tube.

4. A reactance modulating network for the resonant circuit of a radiofrequency oscillator,

said network comprising an electron tube having cathode, anode, and gridelectrodes, couplings from the grid and anode, respectively, to pointsof different potential on said resonant circuit, saidcouplings havingnegligible impedance to the oscillator currents, a phase-shiftingcircuit for applying a potential to the cathode which is phase displacedapproximately with respect to the potential at the anode, and modulatingmeans connected between the grid and cathode for varying thetransconductance of the tube in accordance with a signal potential.

5. A network as set forth in claim 4, wherein said phase-shiftin circuitcomprises reactive and resistive elements connected in series .betweensaid points, said cathode being connected to the junction of saidelements.

6. A reactance modulating network for the resonant circuit of a radiofrequency oscillator, said network comprising an electron tube havingcathode, anode, and grid electrodes, a cou-v pling of negligibleimpedance to. oscillator currents from the anode to one point on saidresonant circuit, a second coupling between the grid and another pointon said circuit, a phase-shifting circuitfor applying a potential fromsaid circuit to the cathode which is phase displaced approximately 90with respect to the potential applied to the anode, and modulating meansconnected between the grid and cathode for varying the transconductanceof the tube in accordance with a signal frequency potential, theimpedance of said second coupling being negligible to oscillatorcurrents, but high to said signal frequency potential.

'7. A network as set forth in claim 6, wherein said second coupling is ablocking condenser.

8. A network as set forth in claim 6, wherein said phase-shiftingcircuit comprises reactive and resistive elements connected in seriesbetween said points, said cathode being connected to the junction ofsaidelements.

9. A network as set forth in claim 8, wherein said reactive element isconnected between said anode and cathode, and said resistive element isconnected between said cathode and grid.

10. A network as set forth in claim 6, wherein the Q of saidphase-shifting circuit is low.

11. A network as set forth in claim 6, wherein said phase-shifting meanscomprises capacitative reactance between the anode and cathode, saidreactance being the capacity between said anode and cathode electrodes.

12. A reactance network for a high frequency circuit, said networkcomprising an electron tube having cathode, anode, and grid electrodes,couplings from the grid and anode, respectively, to points of diflerentpotential on said circuit, said coupings having substantially likeimpedance at said frequency, means for applying a potential to thecathode which is phase displaced with respect to the potentials appliedto said grid and anode, said means including a phase shifter having areactance and a resistance in series between said points of difierentpotential, said reactance comprising the capacity between said anode andcathode electrodes, and means for varying the transconductance of saidtube.

R. RAMBO.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITE) STATES PATENTS Number Name Date 2,144,541 Mayer Jan. 17, 19392,216,829 Plebanski Oct. 8, 1940 2,235,565 Roberts Mar. 18, 19412,279,660 Crosby Apr. 14, 1942 2,323,598 Hathaway July 6, 1943 2,349,811Crosby May 30, 1944

